1. Field of the Invention
This invention relates to the manufacture of compressed wood fiber products, particularly to panel products made from lignocellulosic fibers and commonly termed "hardboard". The invention is particularly concerned with the production of hardboard having some of those properties characteristic in the industry to "tempered" hardboard.
2. Description of the Prior Art
In order to better understand the prior art and the present invention, reference may be made to the accompanying drawing illustrating the schematic flow diagrams of the processes.
In general, the making of hardboard involves reducing wood logs or billets to chips. The chips are subjected to a cooking treatment with steam to soften them, then reduced to fibers by a mechanical refining process in an attrition device such as a disc refiner followed by further mechanical refining in a beater, Jordan refiner or the like. Alternatively, increasingly high steam pressures up to about 1,200 p.s.i. may be used on the chips, followed by a quick depressurization to disintegrate the chips into fibers and fibrille fractions. The prior art has then provided a couple of different basic processes for felting the fiber into hardboard, the wet process and the dry process. In the wet felting processes, the fibers are formed into an aqueous slurry of suitable consistency which is chemically treated as with binding oils or resins, sizes, pH adjusting chemicals and the like in a stock chest and the slurry is passed to the drainage wire of a forming machine in order to produce a wet mat by drainage of the aqueous suspension and partial further dewatering by cold roller pressing. The mat may then be wet pressed or dry pressed, but generally the cold pressed wet mat is pressed against a wire screen (as against a patterned top caul plate where a surface embossing is desired) and further partially dried in the case of smooth on both sides (S-2-S) product; where upon the mat goes into hot air dryers to reduce the moisture content to a low value (e.g. less than 1%). The dried mat is then hot pressed at high temperature, e.g., 400.degree.-600.degree. F. and high pressure, e.g., about 500-1,000 p.s.i. at short cycle times; oven baked and rehumidified in humidifying chambers, and then constitutes the final product for industrial uses or optionally it may be further fabricated and decoratively finished, e.g., as by various coatings and cutting on the surface for a decorative patterned hardboard.
Dry fiber felted hardboard follows the same general process sequence as above except the fiber furnish is dried after preparation from the wood chips and before chemical treatment and mat formation. Fiber handling and mat forming techniques, of course, differ from wet process methods in that the fiber is handled in air and not in water. The present invention is concerned with a wet forming and dry pressing form of process referred to above, and provides for the production of high quality hardboard in a wet felting process. The resulting hardboard by the above processes, while possessing many excellent properties, has in the past suffered from low strengths and high moisture take-up. These hardboards will absorb water which reduces the strength of the board and frequently causes appreciable dimensional changes in the board. The attendant swelling and shrinkage causes destruction of the boards at their edges, reduces racking strength and may result in usage disformation. In the past these problems have generally been handled by "tempering-baking" the finally produced standard hardboard. Historically, oil tempered boards have been manufactured by dipping the pressed hardboard in a large tank of drying oil, leaving the hardboard for a short interval of time in the oil to permit absorption, and thereafter baking the oil soaked board in an oven at a temperature within the range of about 300.degree.-450.degree. F. for about 1-6 hours in order to set or stabilize the oil to a resin-like material. Alternatively the oil may be added by roller coating or spraying instead of dipping in these conventional manners of manufacture, an additional 1% to 3% by dry weight of the board of a drying oil is absorbed and then oxidized by the lengthy high temperature baking. This requires considerable monetary outlay for the installation and maintenance of additional equipment, and frequently the tempering oil does not become uniformly distributed throughout the board or uniformly distributed from one board to another.
One prior art process, generally to show the state of the art for producing regular hardboard by the wet process, is shown in U.S. Pat. No. 4,009,073 for a closed water system to produce an increased concentration of sugars on the mat surface, and employs conventional oven drying followed by pressing preferably at higher than ordinary temperatures, e.g., from 450.degree. to about 500.degree. F.; and in the case of "tempered" hardboard the additional steps of further adding 2 to 10 lbs. per thousand square feet of surface area of an oxidizable resin or oil. The treated board is then heat treated in an oven for 21/2 to 4 hours at 280.degree. F. to assist in curing the tempering oil. This patent states that this baking treatment for tempering will also improve physical properties of nontempered board as well.
Further, U.S. Pat. No. 2,978,382 proposes an alternative tempering process in which the additional drying oil is suspended in a water slurry and applied to the wet mat after partial dewatering. The mat is subsequently subjected to the conventional hot pressing operation and to the conventional baking operation to set the drying oil to a resin-like material. U.S. Pat. No. 3,056,718 proposes a further variation using a non-conjugated drying oil after partially dewatering the mat and before drying of the board. This is done to eliminate the wire screen marks in the production of S-2-S hardboard. In this instance a spray of oil is directed onto the wet mat which then again travels into a dryer for drying with hot air at a temperature of about 300.degree. F. to about 750.degree. F. and is finally consolidated by heated hydraulic press operation at about 1,000 p.s.i. pressure and a temperature of 240.degree. C. (464.degree. F.).
Also, although only remotely related, the following patents should be mentioned. U.S. Pat. No. 2,721,504 during mat formation and before final pressing suggests the addition of a drying oil which will then partially cure during pressing for an improvement in an S-1-S hardboard; U.S. Pat. No. 153,749 mentions the addition of linseed oil soaking for a roofing paper to impart weather resistance to that paper; U.S. Pat. No. 2,423,214 discloses the treatment of paper-making furnish by the addition of a linseed oil aqueous emulsion added to the stock chest in the manufacture of ground wood printing paper; and U.S. Pat. No. 3,455,779 discloses the addition of alum to a wood fiber slurry prior to mat formation followed by the addition of an alkali metal salt of tall oil to the treated slurry in order to improve water absorption dimensional changes of the resultant hardboard.